1. Field of the Invention
The present invention relates to a method for separating at least one alkylnaphthalene from a stock mixture containing at least two alkylnaphthalenes, and particularly, relates to a method using an optically active chromatographic packing which is capable of recognizing differences in stereostructure between alkylnaphthalenes in the stock mixture containing monoalkylnaphthalenes and/or dialkylnaphthalenes.
2. Description of the Related Art
A useful alkylnaphthalene is 2,6-dimethylnaphthalene which is a starting material for polyethylene naphthalate having excellent heat resistance and gas barrier characteristics. Production of 2,6-dimethylnaphthalene includes separation of 2,6-dimethylnaphthalene from a feed stock primarily containing dimethylnaphthalene isomers. Various methods for separating the isomers have been proposed.
Among them, many separation methods using X- or Y-zeolite have been disclosed in, for example, U.S. Pat. Nos. 3,133,126, 3,114,782, 3,772,399, 3,840,610 and 4,014,949, and Japanese Patent Publication Nos. 7-29951, 52-945 and 49-27578. In these conventional methods, separation is performed based on a molecular sieve effect. That is, individual isomers have different adsorbabilities to micropores of several angstroms in zeolite by means of different molecular sizes which are caused by the difference in the position of the substituent or substituents. The isomers are separated using an eluent by a difference in solubility.
Thus, in the separation using zeolite, control of the micropore size of the zeolite has important significance. Such control has been performed by metallic cation substitution of zeolite having ion-exchange capacity. The micropore size and metallic cation substitution, however, often lack uniformity. Thus, constant separation of isomers has been performed with difficulty and low efficiencies.